Call Admission Control

Call admission control enables you to control the audio quality and video quality of calls over a wide-area (IP WAN) link by limiting the number of calls that are allowed on that link at the same time. For example, you can use call admission control to regulate the voice quality on a 56-kbps frame relay line that connects your main campus and a remote site.

Audio and video quality can begin to degrade when too many active calls exist on a link and the amount of bandwidth is oversubscribed. Call admission control regulates audio and video quality by limiting the number of calls that can be active on a particular link at the same time. Call admission control does not guarantee a particular level of audio or video quality on the link, but it does allow you to regulate the amount of bandwidth that active calls on the link consume.

Call admission control operates by rejecting a call for bandwidth and policy reasons. When a call gets rejected due to call admission control, the phone of the called party does not ring, and the caller receives a busy tone. The caller also receives a message on their phone, such as "Not enough bandwidth."

Without call admission control, customers may perceive that IP voice is low in quality and unreliable. With call admission control, customers experience situations similar to the time-division multiplexing (TDM) processing and realize that they need more bandwidth for peak hours.

This section describes two types of call admission control that you can use with Cisco Unified Communications Manager:

You can choose either of these methods of call admission control, but you cannot combine them in the same Cisco Unified Communications Manager system. If your system does not contain IP WAN links with limited available bandwidth, you do not have to use call admission control.

Cisco Unified Communications Manager also supports Resource Reservation Protocol (RSVP), an additional CAC mechanism that offers additional capabilities for full-mesh network topologies and for large clusters. See "Resource Reservation Protocol" for a description of RSVP.

Locations

The locations feature, available in Cisco Unified Communications Manager, provides call admission control for centralized call-processing systems. A centralized system uses a single Cisco Unified Communications Manager cluster to control all the locations. Figure 8-1 illustrates call admission control that is using locations. For more information, refer to the "Location Configuration" section in the Cisco Unified Communications Manager Administration Guide and to Cisco Unified Communications Solution Reference Network Design (SRND) Based on Cisco Unified Communications Manager. For non-centralized systems, Cisco Unified Communications Manager offers an alternative CAC method, Resource Reservation Protocol (RSVP). See "Resource Reservation Protocol" for a description of RSVP.

Figure 8-1 Call Admission Control Using Locations in a Centralized System

In a centralized call-processing system, as illustrated in Figure 8-1, the Cisco Unified Communications Manager cluster resides at the main location, along with other devices such as phones and gateways. The remote locations (for example, branch offices of your company) house additional phones and other devices, but they do not contain any call-processing capability. The remote locations connect to the main location by means of IP WAN links (and possibly PSTN and ISDN links as backups) and to each other by going through the main location (central campus).

Calls between devices at the same location do not need call admission control because those devices reside on the same LAN, which has unlimited available bandwidth. However, calls between devices at different locations must travel over an IP WAN link, which has limited available bandwidth.

The locations feature in Cisco Unified Communications Manager lets you specify the maximum amount of audio bandwidth (for audio calls) and video bandwidth (for video calls) that is available for calls to and from each location, which thereby limits the number of active calls and limits oversubscription of the bandwidth on the IP WAN links.

Note Each audio call includes two streams, one in each direction. Video calls have four or six streams (that is, two or three streams in each direction).

For example, assume that you have configured the following locations in Cisco Unified Communications Manager Administration:

Location

Bandwidth (kbps)

San Francisco (main location)

Unlimited

Austin (remote location)

160

Dallas (remote location)

200

Cisco Unified Communications Manager continues to admit new calls to a link as long as sufficient bandwidth is still available. Thus, if the link to the Austin location in the example has 160 kbps of available bandwidth, that link can support one G.711 call at 80 kbps (in each direction), three G.723 or G.729 calls at 24 kbps each (in each direction), or two GSM calls at 29 kbps each (in each direction). If any additional calls try to exceed the bandwidth limit, the system rejects them, the calling party receives reorder tone, and a text message displays on the phone.

When you configure a location in Cisco Unified Communications Manager Administration, you assign it a name and maximum audio bandwidth. If you set the audio or video bandwidth to Unlimited, you allocate unlimited available bandwidth and allow an unlimited number of active calls on the IP WAN link for that location. In configuring a location, you also assign a video bandwidth for the location. If you set the video bandwidth setting to None, no video calls can connect between this location and other locations, but they can take place within this location.

When you configure a phone or other device in Cisco Unified Communications Manager Administration, you can assign it to a location. If you set the location to Hub_None, you assign that device to an unnamed location with unlimited available bandwidth and allow an unlimited number of active calls to and from that device.

Location reservations move to reflect the type of call. When a call changes from video to audio-only, the location reservation moves from the video location to an audio location. Calls that change from audio-only to video cause the opposite change of location reservation.

Locations and Regions

Locations work in conjunction with regions to define the characteristics of a network link. Regions define the type of compression (G.711, G.722, G.723, G.729, GSM, or wideband) that is used on the link, and locations define the amount of available bandwidth for the link. You assign each device in the system to both a region (by means of a device pool) and a location. As illustrated in Figure 8-2, the regions and locations can overlap and intersect in various ways, depending on how you define them. For more information, see the "Regions" section on page 5-6.

Figure 8-2 Interaction Among Locations and Regions

Bandwidth Calculations

In performing location bandwidth calculations for purposes of call admission control, Cisco Unified Communications Manager assumes that each call stream consumes the following amount of bandwidth:

Each video call can comprise four or six call streams. For a video call, total bandwidth represents the sum of the call audio bandwidth plus video bandwidth but does not include the call overhead.

The audio bandwidth value that is specified for a location includes overhead, whereas the video bandwidth value that is specified for a location does not include overhead. For a location, the bandwidth that is available for video calls represents the sum of the audio bandwidth and the video bandwidth. Refer to the "Understanding Video Telephony" chapter for more details.

Cisco Unified Communications Manager allows calls to complete over a link until sufficient bandwidth does not exist for a new call. At that point, any additional calls fail, and the calling party receives reorder tone.

When a link to a location experiences blockage, it may result from bandwidth leakage that has reduced the usable bandwidth for the location. You can resynchronize the bandwidth allotment to the maximum setting for the location without restarting the Cisco Unified Communications Manager server. For instructions, refer to "Resynchronizing a Location Bandwidth" in the Cisco Unified Communications Manager Administration Guide.

Note If you resynchronize the bandwidth for a location when calls are using the link, the bandwidth might be oversubscribed until all calls that are using the link disconnect. An oversubscribed link can cause audio and video quality to degrade. For this reason, resynchronize the location bandwidth during hours when the link has low traffic.

Media Termination Point (MTP) and transcoder represent exceptions to the bandwidth rules that are outlined in the preceding paragraph. Calls that are made through an MTP can complete even if they exceed the available bandwidth limit. Calls that are made through an MTP, however, cannot provide video.

Caution In the United States and Canada, routing an emergency 911 call to a link that has no more available bandwidth can block the 911 call. For each location on your network, always route 911 calls to the local public switched telephone network (PSTN) through a local VoIP gateway.

Enhancing locations mean that, when a precedence call arrives and not enough bandwidth can be found to connect the call to the destination location, Cisco Unified Communications Manager finds the call or calls with the lowest precedence level and preempts the call(s) to make sufficient bandwidth available for a higher precedence call. If the bandwidth requirement still cannot be satisfied after going through the preemption procedure, the newly placed call fails.

Gatekeepers and Trunks

A gatekeeper device, the Cisco Multimedia Conference Manager (MCM), provides call admission control for distributed call-processing systems. In a distributed system, each site contains its own call-processing capability. For example, Figure 8-3 shows two sites, each with its own Cisco Unified Communications Manager, that an IP WAN link connects. A gatekeeper provides call admission control over the IP WAN link in this example.

In addition to call admission control, gatekeepers can also perform E.164 address resolution to route calls between sites. For example, in Figure 8-3, the extension range for one Cisco Unified Communications Manager specifies 1XXX and 2XXX for the other. Both register with the gatekeeper for call admission control. Each Cisco Unified Communications Manager incorporates an appropriate entry in its respective dial plan route pattern configuration that points the other Cisco Unified Communications Manager extension number range to the gatekeeper. In practice, when user 1001 dials user 2002, Cisco Unified Communications Manager 1XXX sends 2002 to the gatekeeper for address resolution. If the call satisfies the call admission control criteria, the gatekeeper returns the IP address of Cisco Unified Communications Manager 2XXX to Cisco Unified Communications Manager 1XXX. Using the IP address of Cisco Unified Communications Manager 2XXX, Cisco Unified Communications Manager 1XXX can then complete the call to directory number 2002.

Figure 8-3 Call Admission Control by Using a Gatekeeper in a Distributed System

If the IP WAN is not available in this scenario, the call cannot go through as dialed. To simplify the dial plan and also provide fallback to the PSTN, use 10-digit dialing (or adhere to the national dial plan). For example, under the North American Numbering Plan (NANP), a route pattern of XXXXXXXXXX would direct calls to the gatekeeper for address resolution. If the gatekeeper does not allow the call to go over the WAN, Cisco Unified Communications Manager can add the prefix 91 to the dialed digits to reroute the call through the PSTN.

Sometimes an anonymous H.323 device, a device that is not known to Cisco Unified Communications Manager, tries to initiate (send or receive) calls with Cisco Unified Communications Manager. This anonymous device could be a Cisco IOS product (such as a gateway) or any third-party H.323 device.

You can configure H.323 gateways either with gatekeeper control or locally as gateways. If you are configuring with gatekeeper control, or you want to configure an anonymous H.323 device, perform the following steps:

Gatekeeper and Trunk Configuration on the Router

Recommended platforms for the gatekeeper include Cisco 2600, 3600, 3700, or 7200 routers with Cisco IOS Release 12.1(3)T or higher. When configuring the gatekeeper function on one of these routers, you define a set of zones for call admission control. The unique name for each zone includes the IP address of each Cisco Unified Communications Manager that registers with that zone, the zone prefix (directory number range), and the bandwidth that is allocated for that zone.

Cisco Unified Communications Manager registers with a gatekeeper by using its IP address. You can specify the IP address in one of the following ways:

•In the Technology Prefix field under Device > Trunk in Cisco Unified Communications Manager Administration, enter 1#* and enter the command gw-type-prefix 1#* default-technology on the gatekeeper. When a Cisco Unified Communications Manager registers with the gatekeeper, it sends its IP address and the specified technology prefix to the gatekeeper. The gatekeeper then registers this Cisco Unified Communications Manager as a valid gatekeeper-controlled VoIP device.

You associate the Cisco Unified Communications Manager IP address with a particular zone by performing the following steps:

•Use the zone local command on the gatekeeper to define local zones. Enter the zone name in the Zone field.

•In the Zone field under Device > Trunk in Cisco Unified Communications Manager Administration, enter the zone name. When a Cisco Unified Communications Manager registers with the gatekeeper, it sends its IP address and the specified zone name to the gatekeeper. The gatekeeper then registers each Cisco Unified Communications Manager and associates it with the appropriate zone.

To specify the directory number range for a particular Cisco Unified Communications Manager, you use the zone prefix command to configure the range on the gatekeeper. For example, the following command specifies that the DN for zone LHR ranges from 3000 to 3999.

zone prefix LHR 3...

The maximum number of active calls that are allowed per zone depends on the codec that is used for each call and the bandwidth that is allocated for the zone. With Cisco Unified Communications Manager, G.711 calls request 128 kbps, and G.723 and G.729 calls request 20 kbps. Use regions in Cisco Unified Communications Manager to specify the codec type and use the bandwidth total zone command on the gatekeeper to specify the available bandwidth. For example, the following command allocates 512 kbps to the LHR zone.

bandwidth total zone LHR 512

With an allocation of 512 kbps, the LHR zone in this example could support up to four G.711 calls at the same time.

For more information on programming the gatekeeper, refer to the Configuring H.323 Gatekeepers and Proxies section of the Cisco IOS H.323 Configuration Guide.

You can configure gatekeepers and trunks in Cisco Unified Communications Manager Administration to function in either of the following ways:

Non-Gatekeeper-Controlled Trunks

In this case, you explicitly configure a separate intercluster trunk for each remote device cluster that the local Cisco Unified Communications Manager can call over the IP WAN. You also configure the necessary route patterns and route groups to route calls to and from the various intercluster trunks. The intercluster trunks statically specify the IP addresses of the remote devices. To choose this method, use Device > Trunk and select Inter-Cluster Trunk (Non-Gatekeeper Controlled) in Cisco Unified Communications Manager Administration.

Note For a local non-gatekeeper-controlled intercluster trunk, you must specify the IP addresses of all remote Cisco Unified Communications Manager nodes that belong to the device pool of the remote non-gatekeeper-controlled intercluster trunk.

Gatekeeper-Controlled Trunks

In this case, a single intercluster trunk suffices for communicating with all remote clusters. Similarly, you need a single H.225 trunk to communicate with any H.323 gatekeeper-controlled endpoints. You also configure route patterns or route groups to route the calls to and from the gatekeeper. In this configuration, the gatekeeper dynamically determines the appropriate IP address for the destination of each call to a remote device, and the local Cisco Unified Communications Manager uses that IP address to complete the call.

This configuration works well in large as well as smaller systems. For large systems where many clusters exist, this configuration helps to avoid configuring individual intercluster trunks between each cluster. To choose this method, use Device > Trunk and select Inter-Cluster Trunk (Gatekeeper Controlled)in Cisco Unified Communications Manager Administration.

If you configure gatekeeper-controlled trunks, Cisco Unified Communications Manager automatically creates a virtual trunk device. The IP address of this device changes dynamically to reflect the IP address of the remote device as determined by the gatekeeper. Use trunks when configuring the route patterns or route groups that route calls to and from a gatekeeper.

Gatekeeper and Trunk Configuration Checklist

Table 8-2 lists the general steps for configuring call admission control that is based on gatekeepers and trunks.

Table 8-2 Gatekeeper and Trunk Configuration Checklist

Configuration Steps

Procedures and related topics

Step 1

On the gatekeeper device, configure the appropriate zones and bandwidth allocations for the various Cisco Unified Communications Managers that will route calls to it.

Repeat this step for each Cisco Unified Communications Manager that will register with the gatekeeper. Make sure Host Name or IP Address is set the same way on each Cisco Unified Communications Manager.